Hitherto, color recording techniques for facsimile printers, copying machines, television images, etc., have been desired for meeting these desirements, color recording techniques by electrophotography, ink jet printing, thermal transfer recording, etc., have been investigated.
Since in a thermal transfer recording system, the management and operation of apparatus are easy and the cost for the apparatus and supplies are low, the system is considered to be advantageous as compared to other systems.
As a thermal transfer recording system, there are a melting system of heating a transfer sheet having a heat-melting ink layer formed on a base film by a heating means such as a thermal head, infrared rays, etc., to melt the ink and transferring the molten ink onto a recording material for recording and a sublimation system of heating a transfer sheet having an ink layer containing sublimable dyes formed on a base film by a heating means to sublimate the dyes and transferring the sublimated dyes onto a recording material for recording. In these system, the sublimation system can control the sublimated and transferred amount of dyes by changing the energy given at heating, which facilitates gradation recording, and hence this system is considered to be particularly advantageous.
In the sublimation type thermal transfer recording system, sublimable dyes which are used for the transfer sheet are very important since they give large influences such as the speed of transfer recording, the image quality, storage stability of records, etc., and are required to meet the following conditions.
(1) The dye easily sublimes by the working condition of an ordinary heating means such as a thermal recording head, etc.
(2) The dye does not cause thermal decomposition under the working condition before sublimation.
(3) The dye has a preferred hue for color reproduction.
(4) The dye has a large molecular extinction coefficient.
(5) The dye is stable to heat, light, moisture, chemicals, etc.
(6) The dye can be easily synthesized.
(7) The dye is excellent in ink-making aptitude.